In various pipe arrangements, fittings have been widely used for connecting tubes and connecting a tube to a device. Among them, a detachable type of fitting is frequently preferenced over other types.
One of the detachable types of fittings conventionally used is shown in FIG. 1. In FIG. 1, a reference numeral 1 represents a fitting body, reference numeral 2 an inner ring, reference numeral 4 a box nut, and reference numeral 5 a tube, respectively. One end of the inner ring 2 is forcibly inserted into the tube 5 and the other end of it is forced into an inserting opening 1A of the fitting body 1. Then the box nut 4 is turned and meshed outside the fitting body 1 so that a pressure edge 4A of the nut 4 puts pressure onto an expanded portion 6 of the inner ring 2 to seal the gap between the inner ring 2 and the fitting body 1 and to prevent coming off of the tube 5.
The box nut 4, having a certain length in its axial direction, has a female thread part 4B on its inner circumferential surface and a head portion 4C on its outer circumferential surface, the head portion 4C being used for turning the box nut 4 itself. Usually the head portion 4C is formed into an octagonal or hexagonal shape. In addition, on the box nut 4, both of the female thread part 4B and the head portion 4C are positioned over the same length region in the axial direction.
Prior art showing the above axial positional relation between the female thread part 4B and the head portion 4C is disclosed, for example, in Japanese utility model laid-open (KOKAI) publication No. 2-117494.
For the above-mentioned resin fitting shown in FIG. 1, since high temperature chemicals, such as sulfuric acid, sometimes flow therethrough, sealing performance of the fitting becomes an important factor. However, ill the above-mentioned fitting, there are two places which must be sealed; one is an aperture 7 between the inner ring 2 and the fitting body 1 and the other an aperture 8 between the inner ring 2 and the tube 5. The possibility of leakage will be increased with an increase in the number of sealed places.
The apertures between the tube 5 and the inner ring 2 and between the tube 5 and the inner surface of the inserting opening 1A are the secondary places to be sealed, the tube 5 is mainly held not to come off by the pressure edge 4A of the box nut 4. As a result, when the tube 5 is softened due to the flow of high temperature chemicals, it is difficult for the pressure edge 4A to retain a secure hold and to prevent the tube 5 from coming off.
Axial one end of the inner ring 2 is faced to a passage and pressed by the pressure edge 4A (refer to the left-side end in FIG. 1). Hence, the end may be softened and bent inward because of high temperature liquids flowing therethrough, which will cause increased resistance of the passage and decreased sealing properties.
Further, for the box nut 4, the female thread part 4B and the head portion 4C are formed inside and outside of the same axially-positioned cylindrical portion of the box nut 4, respectively. Thus the thickness of the cylindrical portion differs place to place in its circumferential direction. In detail, the thickness at the center of a flat surface of the head portion 4B (octagon or hexagon in cross section) is thinner than that at the edges of the flat surface. On one hand, resin materials show greater coefficients of thermal expansion and contract at constant coefficents of contraction during a cooling process of injection moulding. In this contraction, changes in thickness of the resin material cause different actual sizes in contraction, even though the contraction rate of a resin material is uniform. Namely, there is a difference in the amount of contraction from place to place in the circumferential direction of the box nut 4. This causes sinking, which sometimes results in deteriorating the roundness of the female thread part 4B. This lack of roundness will lead to nonuniformity of tightening force by the box nut 4, thus causing a situation in which the tube 5 easily comes off or falls off and the aforementioned two apertures 7 and 8 to be sealed are substantially lowered in sealing properties.
As discussed above, the conventional resin fitting has a drawback of lowered reliability.